1. Field of the Invention
The present invention relates to odometers and more particularly to odometers having antireverse and antisubtraction mechanisms.
2. Prior Art
Odometers having antisubtraction mechanisms which are constructed such that the digit wheel of the odometer displays increasing numbers when the drive shaft rotates in an additive direction but does not subtract from the display when the drive shaft is rotated in the opposite direction are widely known. One type of antisubtraction mechanism is a type which uses a one-way clutch such as a resilient pawl, etc. which is interposed between the drive gear and the least significant digit wheel of the odometer. The one-way clutch prevents the transmission of a reversing force of the drive shaft to the digit wheel. Furthermore, such a construction is disclosed in the U.S. Pat. No. 3,667,671.
However, in an odometer constructed with one-way clutches since the transmission torque between mutually adjacent digit wheels is small, it is difficult to completely prevent reverse rotation of the digit wheels by means of only a one-way clutch. In other words, a very small torque is transmitted through the one-way clutch in the reverse direction and the least significant digit wheel can be driven in the reverse direction. If the least significant digit wheel is rotated in the reverse direction, higher digit wheels are also rotated in the reverse direction with the result that subtraction occurs.
Consequently, it is necessary in actual application to add an antireverse mechanism to the one-way clutch mechanism to block the reverse rotation of the least significant digit wheel. Such a conventional antireverse mechanism is disclosed in the U.S. Pat. No. 4,192,450. In this mechanism, a spring plate equipped with a pair of antireverse pawls is utilized. A projection that permits the antireverse pawls to slide over it during the rotation of the digit wheel in the positive direction but blocks the reverse rotation of the digit wheel through engagement during reverse rotation is provided on the side face of the digit wheel.
However, because the antireverse pawls slide over the projections wheel resiliently bending during the rotation of the digit wheel in the positive direction, the innercircumferential surface of the digit wheel is struck by a rebound force that is a reaction to the bending of the antireverse pawls as it returns to the normal state every time the antireverse pawls pass over the projection. As a result, a problem occurs in that the noise is generated at each impact. These impacts are intermitted and echo through the circumferential surface of the digit wheel. The generation of such noise is against the general trend in automobiles and other motor vehicles wherein it is desirable to reduce the noise in the interior of the motor vehicle. Furthermore, because the spring plate in such a prior art antireverse mechanism is provided with a pair of antireverse pawls, the antireverse pawls hit twice in each sequence of rotation and thus create even more noise. In addition, since the spring plate having the antireverse pawls must be inserted onto the shaft by using the fitting between the groove and the projection, the construction becomes complicated requiring more labor and higher cost.